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PAPERJiandi Wan et al.Stabilization of carbon dioxide (CO

2) bubbles in micrometer-diameter

aqueous droplets and the formation of hollow microparticles

Volume 16 Number 9 7 May 2016 Pages 1535–1710

Lab on a Chip

PAPER

Cite this: Lab Chip, 2016, 16, 1587

Received 22nd February 2016,Accepted 16th March 2016

DOI: 10.1039/c6lc00242k

www.rsc.org/loc

Stabilization of carbon dioxide (CO2) bubbles inmicrometer-diameter aqueous droplets and theformation of hollow microparticles†

Tianyi Lu,a Rong Fan,a Luis F. Delgadillob and Jiandi Wan*a

We report an approach to stabilize carbon dioxide (CO2) gas bubbles encapsulated in micrometer-

diameter aqueous drops when water in the aqueous drops is evaporated. CO2-in-water-in-oil double

emulsion drops are generated using microfluidic approaches and evaporation is conducted in the presence

of sodium dodecyl sulfate (SDS), poly(vinyl alcohol) (PVA) and/or graphene oxide (GO) particles dispersed in

the aqueous phase of the double emulsion drops. We examine the roles of the bubble-to-drop size ratio,

PVA and GO concentration in the stabilization of CO2 bubbles upon water evaporation and show that thin-

shell particles with encapsulated CO2 bubbles can be obtained under optimized conditions. The developed

approach offers a new strategy to study CO2 dissolution and stability on the microscale and the synthesis

of novel gas-core microparticles.

Introduction

Manipulation of the dissolution of carbon dioxide (CO2) gasand the stability of CO2 bubbles on the micrometer scale is offundamental importance for many processes. Examples in-clude micro-methanol fuel cells,1,2 CO2 sequestration in po-rous media,3,4 gas exchange in the respiratory system,5 carbonbalance in oceans,6 CO2 enhanced oil recovery,7,8 synthesis offunctional microparticles9–12 and chemical reactions involvingCO2.

13–15 Recent advances in microfluidics have achieved sig-nificant progress in the precise generation and manipulationof microbubbles and emulsion drops,16–20 and thus have beenutilized to study the dynamics of CO2 dissolution and the for-mation of CO2 bubble-based functional microparticles. Themajority of microfluidic approaches for the study of CO2 dis-solution focus on segmented microflows in long serpentinemicrochannels, in which liquid plugs are separated by CO2

gas bubbles.21–24 The CO2 bubbles usually have an initial di-ameter larger than the width of the microfluidic channel andcome into contact with the solid wall via a thin layer of fluid.Although the kinetics of CO2 dissolution can be derived bymeasuring the time-dependent size shrinkage of CO2, the seg-

mented flows of CO2 bubbles affect gas absorption at thegas–liquid interfaces due to the recirculation flow in liquidplugs25 and thus introduce additional complex factors to thedissolution process of CO2, not to mention potential wettingproblems between the bubbles and the channel wall. We havedemonstrated the dissolution of CO2 bubbles with an initialdiameter smaller than the width of the channels and foundthat the dissolution of CO2 bubbles goes through a rapiddissolution regime followed by an apparent equilibrium.26

The observations are explained quantitatively by amulticomponent dissolution model that includes the effectof surface tension and the liquid pressure drop along thechannel. In addition, by taking advantage of the acid–baseequilibria during dissolution of CO2 bubbles, microfluidicapproaches have been developed to produce small CO2 bub-bles (<10 μm).9 Most importantly, the increased acidity nearCO2 bubbles due to CO2 dissolution in microchannels trig-gers the deposition of particles and/or polymers at the bub-ble–liquid interface, leading to significantly increased sta-bility of CO2 bubbles and the formation of microparticleswith encapsulated CO2 bubbles.10–12 These particles haveshown potential applications as drug delivery vehicles andultrasound contrast agents.

While various microfluidic approaches have been devel-oped to explore the dissolution and stabilization of CO2

bubbles on the micrometer scale, less is known about thestability of CO2 bubbles encapsulated in micrometer-diameter aqueous droplets.27 Here, we developed a micro-fluidic approach to generate CO2-in-water-in-oil doubleemulsions and investigated the stability of encapsulatedCO2 bubbles. In particular, we showed that, upon the

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aMicrosystems Engineering, Rochester Institute of Technology, Rochester,

New York, USA. E-mail: jdween@rit.edubDepartment of Biomedical Engineering, University of Rochester, Rochester,

New York, USA

† Electronic supplementary information (ESI) available: Calculations of CO2 dis-solution, microscopy image of CO2-in-water-in-oil drops, change in the drop sizeand CO2 bubble size with evaporation, change in percentages of CO2-containingdrops with evaporation, and typical force–distance curve of hollow particles. SeeDOI: 10.1039/c6lc00242k

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removal of water from the aqueous phase of the drops viaevaporation, the stability of CO2 bubbles increased signifi-cantly in the presence of poly(vinyl alcohol) (PVA) andgraphene oxide (GO) particles, and thin-shell particles withencapsulated CO2 bubbles were formed. The present studythus demonstrates a new approach to stabilize CO2 bub-bles in confined micrometer-diameter droplets and the for-mation of CO2-encapsulated microparticles, which we be-lieve is of interest in many applications including thesynthesis of CO2-based functional microparticles and thin-shell particles, CO2 storage, and the design of multiphasemicro-reactors.

ExperimentalMaterials

CO2 gas (99.97%) was ordered from Airgas. PolyIJvinyl alcohol)(PVA, Mw 13000–23000, 87–89% hydrolysed) and sodiumdodecyl sulfate (SDS) were purchased from Sigma-Aldrichand used as surfactants in the aqueous phase. Graphene ox-ide (GO) was ordered from Graphene Supermarket as dryplatelets. Silicone oil (PDMS, polydimethylsiloxane, 20 cSt)was ordered from Sigma-Aldrich and used as the continuousoil phase. Dow Corning 749 fluid that contains 50 vol% highmolecular weight resin and 50 vol% cyclopentasiloxane wasordered from Dow Corning and used as the surfactant in thePDMS oil phase.

Preparation of GO suspensions

A GO stock suspension with a particle concentration of 6 mgmL−1 was prepared by dispersing 30 mg of graphene oxide(GO) particles in 5 mL of DI water whose pH was pre-adjusted to 12 using a NaOH solution (1 M). The stock GOsuspension was then sonicated using an ultrasonic cleaner(VWR Symphony Ultrasonic Cleaner) with a frequency of 35kHz for half an hour. The particle size of GO in the suspen-sion is approximately 10 to 25 μm examined under a micro-scope. GO suspensions of 0.5 mg ml−1, 1.0 mg ml−1, or 3.0mg ml−1 were prepared by diluting the stock GO suspensionaccordingly.

Preparation of PVA solutions

Aqueous 10 wt% PVA solution was prepared as a stock solu-tion by adding 20 g of PVA to 180 g of DI water. The PVA so-lution, under continuous stirring, was heated at 90 °C over-night to ensure complete dissolution of PVA. PVA solutionsof 4 wt%, 5 wt%, or 6 wt% were prepared by diluting thestock solution accordingly. 5 wt% PVA solution with a variedGO concentration was prepared by adding 1 ml of GO stocksolution and 5 mL of DI water (0.5 mg mL−1 GO), 2 ml of GOstock solution and 4 mL of DI water (1 mg mL−1 GO), or 6 mlof GO stock solution (3 mg mL−1 GO) to a 6 ml PVA stocksolution.

Fabrication of glass capillary microfluidic devices

Microfluidic glass capillary devices, as shown in Fig. 1A, werefabricated based on established protocols.28 Briefly, two cylin-drical glass capillaries (standard glass capillaries 1/0.58 OD/ID mm, World Precision Instruments) with the same innerdiameter of 0.58 mm were tapered using a pipette puller toobtain inner diameters of 160 μm and 320 μm, which wereused as the injection (gas phase) and collection capillaries,respectively. The tapered part of the injection capillary wastreated with n-octyltriethoxysilane. The injection and collec-tion capillaries were fitted into a square capillary that has aninner diameter of 1.05 mm.

Generation of double-emulsion

Innermost gas phase CO2 flows into the injection capillarywith pressure PG (PG = 1.3 psi). The aqueous phase and thecontinuous oil phase flow through the two sides of thesquare capillary with flow rates of Qw = 8–21.2 ml h−1 andQo = 18.2–29.5 ml h−1, respectively. As CO2 arrived at the tipof the tapered capillary, CO2 was encapsulated into aqueousdrops and then the gas phase and aqueous phase were emul-sified into the continuous oil phase in a dripping regime. Sy-ringe pumps (New Era Pump Systems) were used to adjustthe liquid flow rates. CO2 injection pressure was controlledby a pressure regulator (OMEGA, PRG200). A high-speed

Fig. 1 Microfluidic generation of CO2-in-water-in-oil double emulsiondrops and the stability of encapsulated CO2 microbubbles upon waterevaporation. (A) Schematic of the microfluidic setup for the generationof CO2-in-water-in-oil drops. Aqueous phase: DI water with 2.5 wt%SDS; oil phase: PDMS oil with 2 wt% 749. (B) Typical images of thedrops with encapsulated CO2 microbubbles before and afterevaporation at room temperature. t = 0 min is defined as the timewhen the drops are collected outside the microfluidic device. Scalebar: 200 μm. Inset: schematic of a CO2-in-water-in-oil drop in thepresence of SDS. (C) The change in percentages of CO2-containingdrops with evaporation time at different initial bubble-to-drop diame-ter ratios. dt=0 and Dt=0 are the diameters of the bubble and drop(shown in inset), respectively, when they were collected outside themicrofluidic device.

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camera (Phantom M120) coupled to an optical microscope(IN480T-FL, Amscope) was used to monitor the generationprocess of CO2-in-water-in-oil double emulsions. The continu-ous oil phase was PDMS oil with 2 wt% 749 fluid. The aque-ous phase was DI water with 1–2.5 wt% SDS. To examine theeffect of PVA and GO on the stability of encapsulated CO2

bubbles upon evaporation, PVA and GO suspensions with var-ious concentrations were added to the aqueous phase.

Evaporation of emulsion drops

CO2-in-water-in-oil double emulsion droplets in a continuousoil phase (∼8 μL) were collected on a glass slide and theaqueous phase of the drops was allowed to evaporate at roomtemperature for 120 min. To monitor the evaporation pro-cess, a microscopy image of the double drops was recordedevery 10 min using the high-speed camera (Phantom M120).ImageJ was used to analyze the size of the bubbles anddrops.

Scanning electron microscopy

Particles generated from double emulsion droplets uponevaporation were washed with isopropyl alcohol, and thendried and coated with a thin layer of gold. The particles werecharacterized by SEM (Amray 1830) at an accelerating voltageof 20 kV.

Atomic force microscopy

Contact mode measurements were performed with an MFP-3D atomic force microscope (Asylum Research, SantaBarbara, CA). Silicon nitride AFM cantilevers (Asylum Re-search, Santa Barbara, CA) with spring constants in the rangeof 30 pN nm−1 were used to indent into a single hollow parti-cle. To prevent the spheres from shifting during indentation,the particles were adhered to a glass slide with an ultravioletcuring optical adhesive (Norland Products, Cranbury, NJ).The cantilever was positioned over the top of a single particleand indented with a loading rate of 2 μm s−1 and a maximumindentation force of 5 nN. Force and distance curves were cal-culated by multiplying the cantilever deflection with thespring constant for the force curves and by subtracting thecantilever deflection from the height position for the distancecurves. Results were obtained based on measurements fromseven different particles and at different measuring positions.

Results and discussionStability of encapsulated CO2 bubbles in the presence of SDS

We utilized a glass capillary microfluidic device to generateCO2-in-water-in-oil double emulsion drops and investigatedthe stability of CO2 microbubbles upon evaporating the aque-ous phase (Fig. 1A). In particular, DI water with SDS andPDMS oil with 749 fluid as surfactants were used as the aque-ous and continuous oil phases, respectively. After CO2-in-water-in-oil double emulsion drops were generated, the sizeof CO2 bubbles decreased quickly inside the microfluidic

channel and reached a constant size after being collected out-side the microfluidic device (Fig. S1†). The stabilization ofthe size of CO2 bubbles in drops was likely due to saturationof the aqueous drops with dissolved CO2. Indeed, the actualconcentration of dissolved CO2 in our experiments was closeto the estimated concentration of dissolved CO2 based onHenry's law (Table S1†).

We then studied the stability of CO2 bubbles when theaqueous phase was allowed to evaporate for 120 min at roomtemperature. Fig. 1B shows the typical images of drops beforeand after evaporation. Evidently, the majority of CO2 bubblesdisappeared after water was removed and hollow particleswere formed. The shell of the particle was presumably com-posed of SDS and the non-volatile, polymeric components of749 fluid. When we changed the size of bubbles (d0) anddrops (D0) and studied the effect of size ratio (d0/D0) on thestability of CO2 bubbles, the percentage of drops with encap-sulated bubbles decreased with time for all three size ratios(d0/D0 = 0.55, 0.46 and 0.34) (Fig. 1C). Particularly, the per-centage of drops with the highest size ratio (d0/D0 = 0.55) de-creased the fastest and the drops with a mid-range size ratio(e.g., d0/D0 = 0.46) had slightly better performance than thedrops with the two other size ratios at the end ofevaporation.

The observed phenomena might be due to an interfacialinstability at the early stage of evaporation and the effect of asolid shell at the late stage of evaporation. Because the sizeof drops became smaller as the aqueous phase evaporated(Fig. S2†), the surface area of the drops decreased. As a re-sult, the concentration of SDS and surfactant 749 at the aque-ous–oil interface increased. This non-equilibrium absorbedexcess of surfactants can trigger an interfacial instability andlead to explosion of drops.29 When the thickness of the aque-ous layer between bubbles and drops was small (the bubble-to-drop size ratio was large, e.g., d0/D0 = 0.55), encapsulatedbubbles encountered this instability at the early time of evap-oration and burst, resulting in a quickly decreased percent-age of drops with encapsulated bubbles, as observed in Fig.1C. On the other hand, when the thickness of the aqueouslayer between bubbles and drops was large (the bubble-to-drop size ratio was small, e.g., d0/D0 = 0.34), the bubbles wererelatively stable at the early time of evaporation (Fig. 1C).However, when a sol–gel transition of the aqueous layer oc-curred in the middle of evaporation, the viscous liquid in theaqueous layer transformed into a solid shell with a relativelylarge thickness due to the small d0/D0. Although the thickelastic shell of a capsule had a large threshold buckling pres-sure and was stable against shell deformation during waterevaporation,30,31 the shell would also exert large stresses tothe encapsulated bubbles if the size of the bubbles kept in-creasing, which would lead to bubble instability. Indeed, in-creased bubble size upon evaporation was observed (Fig. S3†),presumably due to the diffusion of gases (e.g., O2 or N2) fromthe surrounding environment.26,27 As a result, the stability ofbubbles with a small d0/D0 at the later stage of evaporation de-creased quickly (Fig. 1C). The relatively high percentage of

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drops with stable encapsulated bubbles for a mid-rangebubble-to-drop size ratio (e.g., d0/D0 = 0.46) (Fig. 1C) waslikely due to the minimized effect of interfacial instability atthe early stage of evaporation and a relatively low bucklingpressure of the shell at the later stage of evaporation.

Effect of polyIJvinyl alcohol) (PVA) on the stability of CO2

bubbles upon evaporation

Because PVA is an effective emulsifier and can stabilize wa-ter–oil or gas–water interfaces and increase the viscosity ofthe aqueous phase,28,32,33 we tested the effect of PVA on thestability of CO2 bubbles upon evaporation. Particularly, weadded 4 wt%, 5 wt%, or 6 wt% PVA to the aqueous drops inthe presence of SDS. Fig. 2A shows the typical images ofdrops with 4 wt% PVA upon evaporation. The percentage ofdrops with encapsulated bubbles increased significantly afterevaporation (Fig. 2B) compared to that of drops in the ab-sence of PVA (Fig. 1B). This observation held true for all threedifferent bubble-to-drop size ratios and followed the samepattern observed in Fig. 1C where the percentage of dropswith a mid-range size ratio was the highest. In addition, thepercentage of drops with encapsulated bubbles increasedwith the increase of PVA concentration (Fig. 2C & D) andreached the highest percentage of 96% when 6 wt% PVA wasused for d0/D0 = 0.36 (Fig. 2D). Because the bulk viscosity ofdrops increased due to the addition of PVA, the increasedinterfacial area of drops upon water evaporation could bepresented as folding or deformation of the interface insteadof explosion.29 As a result, CO2 bubbles in PVA-containing

drops were expected to be more stable than those in PVA-freedrops. Indeed, the deformation (or buckling) of the PVA-containing shell was clearly observed at t = 120 min (Fig. 2A).

Effect of graphene oxide (GO) on the stability of CO2 bubblesupon evaporation

GO particles have been used as colloidal surfactants to stabi-lize interfaces including CO2 gas bubbles in aqueousphases,34,35 and play roles in controlling gas diffusion.36,37

We thus hypothesize that adding GO will further stabilizeCO2 bubbles in drops upon evaporation. To test this hypothe-sis, we added GO at a concentration of 1 mg ml−1 to theaqueous phase in the presence of SDS and 5 wt% PVA.Fig. 3A shows the typical microscopy images of CO2-in-water-in-oil emulsion drops before and after water evaporation inthe presence of GO. 94% of drops still contained CO2 bubblesafter 120 min of evaporation. In comparison, the highest per-centage of drops with encapsulated CO2 bubbles was 81%when GO was absent in the aqueous drops (Fig. 2C). Remark-ably, hollow particles with a relatively thin shell around CO2

bubbles were formed and some of the particles exhibited ahexagonal shape (Fig. 3A). The thickness of the shell wasmeasured to be 23.4 ± 9.2 μm. This observation wascompletely absent when drops were evaporated without GO(Fig. 1B and 2A). Adding filler particles such as GO into poly-mer matrices can control the stress distribution in the poly-mer matrix, transfer applied stress from the matrix to theparticle, and thus increase the strength of the polymer ma-trix.38,39 As a result, the mechanical stress arising from the

Fig. 2 Effect of polyIJvinyl alcohol) (PVA) on the stability of CO2 microbubbles upon water evaporation. (A) Images of drops with encapsulated CO2

microbubbles at different times of evaporation. The aqueous phase contains 1 wt% SDS and 4 wt% PVA. The PDMS oil phase contains 2 wt% 749.dt=0/Dt=0 = 0.4. Inset: schematic of a CO2-in-water-in-oil drop in the presence of SDS and PVA. Scale bar: 400 μm. Change in percentages ofCO2-containing drops with evaporation time at different initial bubble-to-drop diameter ratios in the presence of (B) 4 wt%, (C) 5 wt% and (D) 6wt% PVA.

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expansion of bubbles upon evaporation was likely distributedevenly on the PVA–GO composite, resulting in a shell with arelatively homogeneous thickness. Increased GO concentra-tion such as 3 mg ml−1, however, compromised the stabilityof encapsulated CO2 bubbles (Fig. S4†), presumably due tothe poor dispersion of GO at high concentrations.

We further characterized the particles using SEM(Fig. 3B and C) and found that the surface of the particleshad layered structures and was relatively smooth (no signifi-cant deformation, for example). Because the elastic modulusof the shell determined by AFM was 2.85 ± 1.2 MPa (Fig.S5†), the threshold buckling pressure (Pbuckling) could be esti-mated to be 97.9 kPa using the shell theory

,40 where E (= 2.85 ± 1.2 MPa) and

v (≈1/3) are the Young's modulus and Poisson's ratio of theshell, respectively; and h (= 23.4 ± 9.2 μm) and R (= 140 ± 6.8μm) are the thickness and radius of the shell, respectively.Thus, a relatively large capillary pressure will be needed todeform the shell, which explained the observed smooth sur-face of the particles. Note that the calculated Pbuckling wasclose to a recently reported value of 95.8 kPa in a polyIJDL-lactic-co-glycolic acid) (PLGA) shell with encapsulated CO2

gas.27

To further identify the roles of GO in stabilizing encap-sulated CO2 bubbles, we did the same experiments in the

absence of PVA. The results showed that, after evaporatingthe aqueous phase, the percentage of drops with encapsu-lated CO2 bubbles was high (95–98%) and did not changesignificantly compared to that in the presence of PVA(Fig. 4A and B), suggesting that GO (along with SDS) canstabilize encapsulated CO2 bubbles upon evaporation. Wedid, however, find that, in the absence of PVA, CO2 bub-bles started to extrude from the drops, and acorn-shapedparticles with encapsulated CO2 bubbles were formed atthe later stage of evaporation (Fig. 4C and D). Because PVAacted as the polymer matrix in the PVA–GO compositeshell, the absence of PVA significantly impaired thestrength of the shell (or linkage between GO particles),39

and consequently resulted in bubble extrusion when bub-bles expanded upon evaporation. Similar acorn-shaped parti-cles have been observed in phase separation-induced aniso-tropic polymeric particles where polymer–solvent (andpolymer) interactions and interfacial energy play a keyrole.41,42 In our case, it was also likely that the decreasedinterfacial tension between CO2 and the oil phase in thepresence of GO contributed to the formation of acorn-shaped particles.

Fig. 3 Evaporation-induced formation of hollow particles in thepresence of graphene oxide (GO). (A) Images of drops withencapsulated CO2 microbubbles before and after evaporation at roomtemperature. The aqueous phase contains 1 wt% SDS, 5 wt% PVA and1 mg ml−1 GO. The PDMS oil phase contains 2 wt% 749. Inset:schematic of a CO2-in-water-in-oil drop in the presence of SDS, PVAand GO. Scale bar: 700 μm. Inset: microscopic image of a particlewith hexagonal morphology. Scale bar: 100 μm. (B) SEM image of asingle hollow particle. Scale bar: 50 μm. (C) High-magnification SEMimage of the shell. Scale bar: 5 μm.

Fig. 4 Effect of graphene oxide (GO) on the stability of CO2

microbubbles encapsulated in aqueous drops in the absence of PVA.(A) and (B) show, respectively, the change in percentages of CO2-containing drops with evaporation time with and without PVA. (C)and (D) are, respectively, the microscopic images of drops withencapsulated CO2 microbubbles after evaporation (t = 120 min) atdt=0/Dt=0 = 0.53 and dt=0/Dt=0 = 0.38. The aqueous phase contains 1mg ml−1 GO and 2 wt% SDS. The PDMS oil phase contains 2 wt% 749.Inset: schematic of a CO2-in-water-in-oil drop in the presence of SDSand GO. Scale bar: 500 μm.

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Conclusions

In summary, we demonstrated a new approach to stabilizeCO2 bubbles confined in micrometer-diameter aqueous drop-lets and unveiled the roles of PVA and GO in the stabilizationof encapsulated CO2 bubbles upon water evaporation. In addi-tion, by optimizing the bubble-to-drop size ratio, PVA and GOconcentration, thin-shell microparticles with encapsulated CO2

bubbles were observed. Although microfluidic approaches forthe synthesis of gas-core particles have been demonstrated pre-viously,18,28,30 the evaporation-induced formation of particlesusing diffusive gases such as CO2 as the gas core is less stud-ied. Thus, the developed approach not only provides new in-sights into the stability of CO2 bubbles but also paves a newway to produce gas-core particles, which are known to havemany applications in a wide range of fields.

Acknowledgements

The authors gratefully acknowledge the support from theRochester Institute of Technology for Jiandi Wan. Jiandi Wanalso acknowledges the Donors of the American Chemical Soci-ety Petroleum Research Fund for partial support of thisresearch.

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